Isobutene Polymerization Using Chelating Diboranes: Polymerization in Aqueous Suspension and Hydrocarbon Solution

The use of the chelating diboranes o-C6F4[B(C6F5)2]2 (1) and o-C6F4(9-BC12F8)2 (2: 9-BC12F8 = 1,2,3,4,5,6,7,8-octafluoro-9-borafluorene) for the polymerization of isobutene (IB) in aqueous suspension or in hydrocarbon solution was studied. Polymerizations in aqueous suspension provided polymer of moderate MW and at variable conversion and were dependent on temperature, mode of diborane addition, the presence of surfactant, and the acidity of and nature of the anion present in the aqueous phase. The T dependence of MW over the T range −80 to −20 °C was studied in aqueous suspension, and higher MW polymer was formed at lower T. The hydrolysis and methanolysis of diboranes 1 and 2 was studied by NMR spectroscopy. Reactions of diborane 1 with excess MeOH or water afford solutions containing oxonium acids [o-C6F4{B(C6F5)2}2(μ-OR)][(ROH)nH] (7: R = H, n > 2; 3: R = Me, n = 3). When diborane 1 is present in excess over water or MeOH, degradation of the diborane is observed. In this case the products are o-C6F4{B(C6F5)2}H (5) and (C6F5)2BOH 7 or (C6F5)2BOMe 4, respectively. In the case of diborole 2, o-C6F4(9-BC12F8)B(2-C12F8-2′′-H)(μ-OH)·7H2O (17) and o-C6F4(9-BC12F8)B(2-C12F8-2′′-H)(μ-OMe) (11) were isolated from reactions of 2 with water and MeOH, respectively, and were characterized by X-ray crystallography. None of these degradation products effect IB polymerization in aqueous suspension. As a model for initiation of polymerization, the reaction of diborole 2 with 1,1-diphenylethylene (DPE) was studied. Addition of MeOH at low T results in efficient formation of the ion-pair [Ph2CMe][o-C6F4(9-BC12F8)2(μ-OMe)] via protonation of DPE. Polymerizations in hydrocarbon media were exothermic and rapid and gave quantitative yields of polymer even at very low concentrations of diborane 1. The T dependence of MW was studied in hydrocarbon solution and showed non-Arrhenius behavior. This was explained by competitive chain transfer to monomer at elevated T and chain transfer to molecular water at lower T.